hammer: Enable LOW_POWER_IDLE
hammer: Enable LOW_POWER_IDLE

Lower hammer's power usage when USB is not in use.

One thing that is missing is some way to put the trackpad into
low power mode as well (should be some I2C command).

BRANCH=none
BUG=chrome-os-partner:62325
TEST=build and flash hammer, connect USB, check power consumption
     averaged over 10s is around ~57 mW (~32 mW for base EC,
     ~25 mW for trackpad).
TEST=Suspend host, power consumption drops to ~3 mW
     (~0.04 mW average for base EC, ~3 mW for trackpad).
TEST=Pressing keyboard key or touch the trackpad wakes the system.

Change-Id: I45f5e300d9d9e2bf49a327949649b0e44b544640
Reviewed-on: https://chromium-review.googlesource.com/446220
Commit-Ready: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Tested-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec